Improvie clarity on auto generated physical resource name

[aureq]

File: content/docs/concepts/resources/names.md

As noted in pulumi/pulumi-azure-native#3406, Azure Native drops the - in the physical resource name.
This is somewhat confusing as the docs give the impression auto-generated names would keep their -. However, this is actually dependent of the provider's own implementation, which is itself also dependent on the cloud vendor.

Adding a note would be welcome.